1. Field of the Invention
The present invention relates to a gas sensing apparatus and, mole particularly, to a low-power, normal-temperature type gas sensing apparatus and method whose sensing operation is made using current-voltage characteristics of a sensing layer without a separate heater.
2. Discussion of the Related Art
Generally, a gas sensing apparatus is constructed in such a manner that a heater is formed in a gas sensing device which directly senses gas. Therefore, when a voltage is applied to the heater, heat is generated to heat a sensing layer of the sensing device to a high temperature (200.degree.-500.degree. C.). The reason to heat the sensing layer using the heater formed in the gas sensing device will be explained below.
The sensing layer of the gas sensing device is formed of a semiconductor oxide as a main material with a metal catalyst like Pt, Au, or Ag added. The sensing operation is performed so that the sensing layer measures the change of the semiconductor oxide's electric conductivity in the presence of gas. The aforementioned semiconductor' oxide has a relatively high resistance (over several M.OMEGA.s) at room temperature. However, in case that the temperature is increased to a relatively high temperature above 200.degree. C., the semiconductor oxide has tens or hundreds of K.OMEGA. in resistance and will react with various types of reducing gas that may be present in the ambient atmosphere.
As described above, a heater has been required in order to make the sensing layer react with the gas and thereby to exhibit a variation in the rate of electric conductivity.
The aforementioned conventional gas sensing device and sensing apparatus will be explained below with reference to the accompanying drawings. FIG. 1 is a cross-sectional view of the conventional gas sensing device. FIG. 2 is a plan view of the conventional gas sensing device.
The conventional gas sensing device is constructed in a manner that a heater 3 for heating a sensing layer and two electrodes 2 for measuring the variation of electric conductivity of the sensing layer are formed on an alumina (Al.sub.2 O.sub.3) substrate 1 by a screen printing method. The sensing layer 4 for sensing gas is formed over the two electrodes 2. At this time, the sensing layer 4 is formed of a semiconductor oxide in which SnO.sub.2 and metal catalyst like Pt, Au, or Ag are mixed. The heater 3 is placed on the back surface of the alumina substrate 1 as shown in FIG. 1, or on the front surface of the substrate 1 on which the electrodes are formed, as shown in FIG. 2.
The conventional gas sensing apparatus which senses gas using the aforementioned gas sensing device will be explained below. FIG. 3 is a block diagram of the conventional gas sensing apparatus.
The conventional gas sensing apparatus is constructed with a gas sensing device 11 shown in FIGS. 1 and 2, a heater driver 12 (heater voltage supply) for driving the heater 3 of the gas sensing device 11, a voltage supply 13 for supplying a voltage to the electrodes of the gas sensing device 11, a current detector 14 for detecting a current that varies in accordance with gas detected by the gas sensing device ill, and a control and judgement portion 15 that controls the voltage supply 13 and heater driver 12, and receives a current detected from the current detector 14 to judge whether or not a gas is present.
The voltage supply 13 and heater driver 12 are controlled by the control and judgement portion 15 so that the voltage supply 13 supplies a voltage to the electrodes 2 of the gas sensing device 11 and the heater driver 12 supplies power to the heater 3 of the gas sensing device 11 to heat the sensing layer 4 to a high temperature i.e., above 200.degree. C. In this state, when the gas sensing device is exposed to gas, the resistance of the sensing layer 4 varies
If the sensing layer 4 is exposed to gas and reacts therewith, the resistance value of the sensing layer 4 is changed into a resistance value lower than that of the sensing layer in air. Therefore, if a uniform voltage is supplied from the voltage supply 13 to the gas sensing device 11, the current is output at a high level. Accordingly, the current detector 14 detects the current which runs through the sensing layer, and applies it to the control and judgement portion 15.
A gas sensing operation is performed so that the control and judgement portion 15 compares the current input from the current detector 14 to measure the electric conductivity of the sensing layer of the gas sensing device 11. That is, in the conventional gas sensing apparatus, a uniform voltage is supplied from the voltage supply 13, and gas detection is accomplished in accordance with the current value detected by the current detector 14.
However, the conventional gas sensing apparatus has several problems. First, the heater and heater driver for heating the sensing layer to a proper temperature are required separately. This complicates the gas sensing apparatus, decreases its productivity, and increases the cost.
Secondly, power consumption is increased because the device is heated to a high temperature. In addition, due to thermal shock, the gas sensing device will deteriorate and may even break causing the reliability of the gas sensing device to be low.
Thirdly, the conventional gas sensing apparatus is capable of sensing the density of gas, but incapable of judging the kind of gas.